<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        body{
            text-align:center;
        }
        canvas {
            border: 1px solid #000;

        }
    </style>
</head>
<body>
<canvas id="cas" width="500" height="500"></canvas>
</body>
<script>

    var cas = document.getElementById('cas');
    var ctx = cas.getContext('2d');

</script>
</html>